Some problems in hot rolling of al-alloys solved by the finite element method.

Abstract

This thesis is focused on employing the finite element method (FEM) to simulate hot flat rolling process. The relevant work involves selecting a suitable constitutive equation, predicting the rolling load and roll torque, computing temperature changes and lateral deformation, simulating the evolution of substructure, modelling static recrystallisation and designing the rolling pass schedule. A practical pass schedule supplied by an aluminium company and containing reliable measured data of roll load and torque is analysed by a commercial 3-D thermornechanical coupled FEM program FORGE3 V5.3. The inverse analysis method is adopted to obtain the friction coefficient and heat transfer coefficient. The distribution of pressure, equivalent strain, the stress and damage in the roll gap in breakdown rolling are discussed. The changes of temperature and lateral profile under both laboratory and industrial rolling conditions are computed and compared with experimental measurements, the differences are then investigated. Through applying the Taguchi experimental design method, the influence of each rolling parameter on the spread, i. e. the ratio of width to thickness, the roll radius to thickness, the thickness reduction, and the deformation temperature, the relative contribution of each control parameter is quantitatively estimated and expressed as a percentage. A new spread formula is built up based on a large amount of FE analyses. The new formula is able to deal with both laboratory and industrial rolling conditions with high accuracy. Critical reviews are presented for the previous work in the modelling of subgrain size and static recrystallisation. Both empirical and physical models are applied to investigate the evolution of subgrain size, dislocation density, misorientation and the flow stress in the roll gap. The predicted subgrain size agrees very well with the experimental measurement. The difference between the use of two models are illustrated and analysed. Studies on modelling of static recrystallisation are carried out by incorporating the plastomechanical parameters, i. e. strain, strain rate and temperature, into empirical model. Various approaches are proposed to reduce the predicted volume fraction recrystallised at the surface and are verified by the comparison with measurement. Simulation results show that some of the previous work reported in the literature are erroneous. Further work in the modelling of static recrystallisation and texture evolution is detailed. The Taguchi experimental method is also applied to study the influence of the rolling parameters on the fraction recrystallised (Xv ). The study shows that rolling temperature has the greatest influence on the Xv, followed by the parameter H. 1L. The roll temperature and roll speed have little influence on the Xv. Designing a rational rolling pass schedule is critical for the control of strip profile and product quality. In the present thesis, the procedure of designing a rolling pass schedule is illustrated. The formulae used in scheduling are listed and explained. The scheduling program is then performed to check with two existing industrial schedules. The comparison shows that the rolling load, temperature and power model is reliable and shows high accuracy. A multipass simulation by the use of finite element method is also carried out and the results are compared with various model predictions. The problems in the simulation are illustrated and explained.